It's 68 degrees Fahrenheit - an unseasonably warm winter's day in London - and Sufjan Stevens has lent me his sunglasses. I know it's 68 degrees because, along with a Maoist khaki cap and his spare shades, he has taken from his jacket pocket a large, flat alarm clock that states the time, date and temperature, and set it beside him on a canalside bench. 'So we're not late getting back,' he says, mindful of a lunch meeting and his preparations for a gig that night which sold out before it had even been advertised.

When you've sold over 100,000 copies of an album released on a label that you run from a single office in the American Midwest, and you manage your own burgeoning career, time is of the essence. Word of Stevens's extraordinary songwriting talent has spread like a prairie fire in the last year, urged on by the 30-year-old's claim that he intends to make a record about each of the 50 states of America.

So far he's made two, the first a sad but uplifting reflection on his home state of Michigan, and the most recent, a fable-filled Broadway musical manque about Illinois. Despite protesting that he'll never complete the task in his lifetime, his fans - who include Elton John, Gwyneth Paltrow and the cast of US indie pop-loving TV drama The OC - are willing him to do the rest. If he does, he'll make history.

Since its release three months ago, on his own Asthmatic Kitty imprint, Sufjan Stevens Invites You to: Come On Feel the Illinoise - to give the record its full, Slade-indebted title - has become one of the most critically acclaimed pop albums in years, debuting at number one on Billboard's 'Heatseekers' chart for upcoming artists and selling 80,000 copies in the US, despite Stevens's refusal to appear on the hype-generating TV shows of David Letterman and Conan O'Brien.

Instead, he's taken his seven-piece 'Illinoisemakers' band, and a show that combines Broadway-style enthusiasm with high-school antics, out to the people. They cartwheel on stage to the sound of a drum roll, wearing home-made orange and blue tracksuits Stevens ran up on his grandmother's Singer, and announce each song with a goofy cheer ('Gimme a J for Jacksonville!' and so on) before teaming up to perform a wobbly human pyramid. A Sufjan Stevens show is a cynicism-free zone.

His willingness to appear eccentric, patriotic and plain dorky belies an intense desire to question the motives of a country for which, he says, he is 'in mourning' due to its dedication to capitalism, harmful industrialisation and soul-killing imperialism. Stevens is building a reputation in Europe as a man of rare talent who, like the Great American Novelists before him, is capable of anatomising that bizarre-seeming country so that it makes sense to us. No other artist of his age seems prepared to take on that role with such dedication.

Reviewers keen to place his idiosyncratic blend of showtunes, jazzy time signatures and uplifting melodies often box him in with other oddly named 'weird folk' auteurs. But where Bright Eyes's Conor Oberst caterwauls about the state of the union with as much subtlety as a hammer tenderising meat, and New Age-y Marc Bolan copyist Devendra Banhart attempts to lure the listening public into believing that his beard has magical powers, Stevens's five albums so far are full of substance, wit and genuine folk artistry.

The Michigan and Illinois albums betray dismay at the downside of the American Dream, a dismay that's no less powerful for being expressed through highly personal vignettes. 'Flint', on the former, details the horrific post-industrial decline of Michael Moore's hometown through the story of a man who, having lost his job, quickly loses his home and any semblance of connection to society. Illinoise's glorious centerpiece, about the Chicago World's Fair of 1893, murmurs 'Oh, what would Frank Lloyd Wright say?'

'Corporations are run on principles of greed, and making the most money possible with the least work possible, and I think that's counterproductive to the health of human beings and society,' says Stevens, a half-confident, half-nervous figure with limpid green eyes who manages to seem distracted and deeply attentive at the same time. 'They create wealth but they destroy cultures and families. The World's Fair was the precursor to theme parks like Disneyworld, and the really sort of cheap, superficial promotional architecture that you see everywhere in the US. I think there's a danger when you start creating a civilisation that isn't meant to last.'

The fact that indictments of western capitalism are sung by a man who appears in public shaking a pair of cheerleader's pom-poms is part of what makes Stevens so appealing.

He wrote many of the lyrics for Illinois after canvassing friends and internet chatroom contributors for their memories, no matter how cheesy, of growing up in the Prairie State. He amassed recollections of hog pageants, Chickenmobiles (a big van with a rooster tail that circles the town of Decatur, plugging a local fast food chain), scout camps and high-school crushes to create a thoroughly convincing narrative.

'I've always thought that everything is worth noting, even the way we speak through anecdotes with our friends,' he says, raising his eyebrows as he sips an evilly strong coffee. 'You know, those small social crises of middle school, like so-and-so just got braces, or Bethany just got her hair crimped, or her bangs were cut too short, or so-and-so started wearing her bra for the first time. The way we like to tell those stories over and over again indicates something about our culture.'

His obsession with the details of ordinary lives makes him sound more like a novelist than a pop star, perhaps understandable given that he moved to New York from Michigan with dreams of becoming a writer. After studying for an MA in creative writing, he worked in publishing before music took over his working life. In many ways, this makes him more like wunderkind writers Jonathan Safran Foer, Dave Eggers and Zadie Smith than the aforementioned weird folkies: he, too, is capable of withering sarcasm but cares too much about human relationships to use irony as his default setting.

Although leavened by a mixture of daft and deadpan humour, Stevens's songwriting is defined by yearning, loss and fear of abandonment. Mother figures get short shrift, unlike big sisters, best friends and God, who often, but not always, give protection and succour. On 'Casimir Pulaski Day', about the death of the narrator's teenage sweetheart from bone cancer, he becomes lost for words and, instead, literally cries a melody.

'Flannery O'Connor said that anyone who survives a childhood has multitudes to write about, because really childhood is a succession of trauma and crisis over and over.'

Stevens refrains from saying as much, but part of that litany of childhood trauma must have come from what he calls his father and stepmother's 'trial-and-error approach' to life, which encompassed changing religious creed every few years to moving house, their six children and a menagerie of pets and farm animals in tow, to wherever it was cheapest to raise their huge family.

Stevens's unusual first name - pronounced 'Soof-yahn' - was given to him by a Subud cult leader whom his parents followed for a time, while his song 'He Woke Me Up Again' recalls the night his dad got everyone out of bed and announced that the family was converting to Catholicism. When Stevens was nine the family moved from run-down Detroit to rural Michigan, where he attended his first state school after spending his early years at a Steiner school, learning to knit and dance before he could read.

'Public school felt like prison - cinderblock walls, fluorescent lights, metal lockers. It was so sterile and unstimulating. Then I went to a Catholic school for a year and then music school for a year. We'd get scholarships or sometimes we'd have to drop out because my parents couldn't pay for it. My stepmother was really interested in alternative education. She hated the public school system.'

His baby-boomer parents' desire to resist conformity led to a strange kind of rebellion: Stevens became a devout Christian. His faith shines through every note of his music - in his wonder at every tiny yet monumental scrap of human endeavour, and in his ultimate (if at times shaky) acceptance of a celestial Big Plan that will rid us of what he has described as 'all the nonsense of suffering' - but it's as far removed from 'Christian rock', with all the deadly dull connotations of that phrase, as it's possible to be.

'I feel like I'm doing a disservice to myself, and to my convictions, in speaking publicly about these things, because they're too easily misconstrued,' he says, bristling at the very thought of elaborating on his faith. 'I find in music there's a space and a language I can use to express things in ways I can't describe conversationally. And it always leads to some kind of discussion about politics. There's a good reason for suspicion of faith, but, you know...' He trails off. 'It's 71.5 degrees. We'd better go.'

I hand him a copy of Joan Didion's Miami, in case he ever reaches Florida in his 50-state musical quest. 'To read Miami is to understand America,' he says, mock-solemnly, reading the blurb. You could say the same about hearing an album by Sufjan Stevens.